Calcite sand
From PLANTFACTS.OSU.EDU
1. A material rich in calcium carbonate, which has the effect of raising the pH of soil as it weathers.

This image is an example of calcite sand crystals.
Source: http://www.museums.udel.edu/mineral/mineral_site/displaycollection/Carbonates/Calcite/9999-0803-0006.html
